Project Summary Circadian clocks have evolved to appropriately align biological processes to the changing 24 h environment. Genetic analyses of circadian locomotor activity rhythms in the fruit fly Drosophila have revealed transcriptional feedback loops as the core organizing principle of circadian clocks. Yet the pace of these circadian feedback loops is largely determined by protein phosphorylation and subsequent degradation, driving rhythmic expression of clock components such as PERIOD (PER). In Drosophila, light is able to reset these oscillators in part via degradation of the clock component TIMELESS (TIM). Remarkably, these clocks are highly conserved among animals. Circadian clocks also enable the appropriate adaptation to seasonal changes in day length or photoperiod. Yet while much is known about both core clock and photic input mechanisms, a mechanistic understanding of how these two pathways collaborate to mediate responses light, including changing photoperiod, is lacking in animals. Here a novel clock component has been discovered, the phosphatase of regenerating liver-1 (PRL-1), that is also important for light mediated resetting and setting behavioral phase under varying seasonal photoperiod. This research proposes to leverage the discovery of PRL-1 to understand how the circadian clock integrates light information to drive appropriately timed behavior. It will specifically address the neuronal basis of PRL-1 function including the role in specific photoreceptor pathways, its function in autonomous and coupling neuronal oscillators, and the role of the light and clock regulated clock component TIM in mediating PRL-1 effects. These studies exploit the discovery of a core clock component with a novel role in photoperiod-dependent behavior. In addition, full advantage is taken of the Drosophila system, including the conservation of the core clock machinery and clock neural network architecture as well as extensive molecular genetic resources to examine gene function in the whole animal. This research also leverages the ability to quantitatively examine molecular oscillations in FACS sorted and intact neurons. This work could provide insights into how circadian clocks integrate environmental information to yield timed behavior.